Cell-based regenerative medicine is a natural extension of developmental biology. Embryonic stem cells (ES cells) give rise to the entire embryo and have the potential to repair diseased tissues in the adult. Cell differentiation is preceded by specification, a process that restricts these pluripotent cells to a particular lineage. Tissue regeneration is dependent on the availability of factors that specify stem cells to the desired lineage and promote their differentiation. Given that the environment of diseased tissues is unlikely to be the same as that of the developing embryo, stem cells may require "conditioning" prior to or during implantation. The overall hypotheses guiding the proposed research are: (1) the fate of pluripotent stem cells can be directed in vitro and in vivo by stem cells previously committed to a particular lineage, and (2) the induction process is mediated by a combination of soluble factors and cell-cell adhesions. This project builds on our published observations that chick epiblast cells expressing MyoD mRNA in vivo recruit pluripotent epiblast cells to the skeletal muscle lineage in vitro by releasing a factor(s) that inhibits the bone morphogenetic protein (BMP) signaling pathway and promotes the expression of N-cadherin. In the absence of MyoD positive epiblast cells, more pluripotent cells form cardiac muscle. A key event in muscle differentiation is the downregulation of E-cadherin. Preliminary studies demonstrate that ablation of MyoD positive cells in the epiblast results in herniation of the organs through the ventral body wall and a reduction in skeletal muscle in the somites. The proposed studies are designed to explore the mechanism whereby MyoD positive epiblast cells promote skeletal myogenesis in the somites in vivo, and to identify the factors that regulate cadherin switching and recruitment of cells to the skeletal and cardiac muscle lineages in vitro. The experiments will focus on the roles of Wnt, 7GF-/3, BMP, and BMP inhibitor family members in these processes. The proposed studies may reveal a novel mechanism for regulating skeletal myogenesis in vivo, and lead to the development of methods for conditioning stem cells to ensure that they are biased to differentiate along the desired pathway and express the appropriate cell-cell adhesion proteins prior to their implantation into diseased tissues.